Problem modeling in perspective view
Perspective, while it is the way we see things in the real world, is an illusion. 

These train tracks look like they are converging, but obviously they can't be.

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Orthographic projection is a way of seeing 3D shapes in 2D at their actual sizes, without perspective distortion.

Here is a comparison of orthographic versus perspective, with the same camera angle.

 

The orthographic version doesn't look "right", but the tracks appear parallel, which is how they actually are.

By moving the camera backwards and increasing the focal length, you can kind of get an ortho-like look.



Assuming you are modeling from an orthographic or nearly orthographic reference image (obviously it is impossible to get a true orthographic projection from a camera, but photographs can be "close enough" if taken with a long focal length), modeling in ortho is the best way to go. It does take a little getting used to though..